In modern mobile telecommunications systems a mobile station may have a choice as to whether to operate with a public cellular system or with an autonomous system, such as a residential system of a private system. Typically it will be desirable to operate with a selected autonomous system, which may provide a more favorable rate structure than the public cellular system(s), or that may provide a desired service not offered by the public cellular system(s). A particular autonomous system may be a residential system that serves the user's home, or a private system that serves the user's workplace.
One such modern cellular system is referred to as IS-136, which is described in IS-136.1, Rev. A, February 1996, and subsequent updated releases. This system employs Digital Control Channels (DCCHS) that enable a mobile station to gain access to the system. When a mobile station scans for and subsequently monitors a DCCH, it is said to be "camped" on that particular DCCH. Page messages and other information are received from the DCCH.
In Section 6.3.19 of IS-136.1there is described a non-public mode search procedure that is to be implemented by IS-136 compliant mobile stations. As defined, while camping on a DCCH a mobile station user may decide to initiate a Non-Public Mode Search Condition in order to search for service with an alternate system (System Identification (SID), Private System Identification (PSID), or Residential System Identification (RSID)) on the current DCCH and/or other DCCHs. Two possible procedures are defined for Non-Public Mode Search: New PSID/RSID Search and Manual System Search.
Discussing first the New PSID/RSID Search procedure, when the user invokes this procedure the mobile station proceeds as follows. First the mobile station collects one signal strength measurement on each frequency in the current frequency band. Next, the mobile station makes a list of up to 24 channels with the strongest signals, and then tunes to the strongest channel in the list. The mobile station then determines if this channel contains a DCCH. If the channel contains a DCCH, the mobile station reads the Fast Broadcast Control Channel (F-BCCH) and determines therefrom if the DCCH is marked with a non-public Network Type (Private and/or Residential) that is enabled in the mobile station. If this is the case, the mobile station marks the DCCH as DCCH.sub.-- 1. If the channel does not contain a DCCH, or if the DCCH is not marked with a non-public Network Type that is enabled in the mobile station, then a determination is made if this is the last channel in the channel list. If it is, the procedure ends, otherwise the mobile station reads the next strongest channel in the channel list and the process repeats.
After possibly performing a Test Registration procedure, and marking an appropriate PSID/RSID as SYS.sub.-- 1, the mobile station displays an indication of SYS.sub.-- 1 to the user. If the user selects SYS.sub.-- 1, and after other processing, a determination is made if DCCH.sub.-- 1 is the current DCCH. If it is not, the mobile station adds DCCH.sub.-- 1 to a list of channels identified as requiring measurements (see Section 6.3.3.1, Control Channel Locking). The mobile station then, after an appropriate delay required for channel measurement purposes (see Section 6.3.3.3), declares a Priority System Condition (see Section 6.2.3) using DCCH.sub.-- 1 as the only reselection candidate. The CELLTYPE for DCCH.sub.-- 1 defaults to PREFERRED until otherwise determined. In addition, the mobile station determines the MS.sub.-- ACC.sub.-- PWR, RSS.sub.-- ACC.sub.-- MIN, SS.sub.-- SUFF and DELAY (see Section 6.3.3.4.2) for DCCH.sub.-- 1 prior to involving or while executing the Control Channel Reselection procedure (see Section 6.3.3).
In the Manual System Search procedure, the mobile station proceeds as follows. The mobile station first searches the current DCCH and neighboring DCCHs (including Private Operating Frequencies (POFs) if they exist) for all candidates that support one or more of the PSIDs, RSIDs, and preferred SID that the mobile station subscribes to. The mobile station then displays a PSID/RSID Alphanumeric Name of each PSID or RSID supported by the candidate control channels that match a PSID or RSID stored in the mobile station, and the Alphanumeric System ID of the preferred SID. The mobile station then marks as DCCH.sub.-- 1 the candidate control channel supporting the SID, PSID, or RSID matching the Alphanumeric System ID or PSID/RSID Alphanumeric Name selected by the user. If more than one candidate control channel supports the selected PSID, RSID, or SID, then the candidate with the highest signal strength is marked as DCCH.sub.-- 1. If no SID, PSID, or RSID is selected by the user, the procedure is simply terminated.
If DCCH.sub.-- 1 is not the current DCCH, then the mobile station performs the same steps as described above for the New PSID/RSID search procedure, i.e., adding DCCH.sub.-- 1 to the list of channels identified as requiring measurements, etc.
In the foregoing procedures the interval of signal strength measurements depends on the values of the information elements SCANINTERVAL and HL.sub.-- FREQ. The SCANINTERVAL information element is received by the mobile station in a Control Channel Selection Parameters message. This message is one of a Layer 3 message set that is received over the F--BCCH. The SCANINTERVAL information element represents a basic measurement interval (expressed in Hyperframes, each having a duration of 1.28 seconds) to be used for each frequency identified as requiring measurement. HL--FREQ is used to modify SCANINTERVAL (if HL.sub.-- FREQ is low then the measurement interval for each frequency is twice SCANINTERVAL). The Control Channel Selection Parameters message also includes a DELAY information element which specifies a period of time that the mobile station must camp on its current DCCH before it can invoke a Candidate Eligibility Filtering procedure (see Section 6.3.3.4.1) in order to locate a new control channel.
In accordance with the foregoing procedures, and giving a worst case example, if SCANINTERVAL is set to 15 (max. value) then the measurement interval is 16 Hyperframes or 16*1.28 sec=20.48 seconds. Since the Reselection procedure requires five signal strength measurements to form the Long.sub.-- RSS (Received Signal Strength) measurement value, the total measurement time becomes 5*20.48=102.4 seconds. Only after this amount of time elapses can the mobile station perform reselection. Furthermore, the Reselection Priority System Condition requires that the mobile station has camped on its current DCCH for a time period greater than that defined by the above-referenced DELAY information element. A maximum specified value for DELAY is 420 Superframes (two Superframes equal one Hyperframe), which implies a maximum time that the user must wait of 420*0.64=268.8 seconds.
While it may appear at first glance that the foregoing problems could be avoided if the serving system simply transmitted small SCANINTERVAL and DELAY values in the Control Channel Selection Parameters message; in practice the use of larger values provides for a reduction in power consumption of the mobile station. As such, there are advantages in not operating the mobile station at the minimum specified SCANINTERVAL and DELAY values.
When the user select selects either the New PSID/RSID Search or Manual System Search it is indicated that the user desires to select a new system at that time, and the user should thus be able to initiate the reselection procedure as soon as possible. As can be appreciated, the currently specified search procedures can result in a significant and user perceptible delay in executing the requested search.